Field of Invention
The present invention relates to a field of calcium sulfate hemihydrate material production method, and more particularly to a method for preparing α-calcium sulfate hemihydrate with calcium sulfate dihydrate.
Description of Related Arts
Calcium sulfate hemihydrate is an air-hardening material, and, after mixing with water, forms hardenite having certain strength. The calcium sulfate hemihydrate is widely applied in different industrial fields. For example, the calcium sulfate hemihydrate can be applied in the ceramic industry for preparing the ceramic mould, in the metallurgical industry for producing the precise casting mould, and in the building industry for producing the various building products and building materials. The calcium sulfate hemihydrate has two crystal forms, respectively α-form and β-form. The calcium sulfate hemihydrate, which is mainly β-form, has poor physical and mechanical properties with a compressive strength generally in a range of 5-15 MPa, while the calcium sulfate hemihydrate, which is mainly α-form, has good physical and mechanical properties with a compressive strength generally in a range of 45-100 MPa.
The calcium sulfate hemihydrate can be prepared through dehydrating the calcium sulfate dihydrate under certain conditions, namely CaSO4.2H2O→CaSO4.0.5H2O+1.5H2O. The production process of the calcium sulfate hemihydrate has the low energy consumption and little environmental pollution, utilizes the solid wastes, and has a good development prospect. The calcium sulfate dihydrate can be natural raw materials and industrial by-product calcium sulfate dihydrate, wherein the industrial by-products are industrial solid wastes. The industrial by-product calcium sulfate dihydrate is mainly from desulfurized slags which are by-products of flue gas desulfurization, phosphorus slags which are by-products of phosphorus chemical industry, citric acid slags which are by-products of citric acid, and titanium slags which are by-products of titanium chemical industry. In China, the annual emissions of the industrial by-product calcium sulfate dihydrate are 130 million tons. Currently, the multipurpose utilization rate is low, and a large amount of industrial by-product calcium sulfate dihydrate is still stacked, which not only occupies the land and pollutes the atmosphere but also seriously pollutes the groundwater. Some industrial by-product calcium sulfate dihydrate having the high quality exists in the industrial by-products and is able to be comprehensively utilized as the high-quality industrial resources.
Three main methods for producing α-calcium sulfate hemihydrate are described as follows.
(1) Autoclave Method
Crushing the natural calcium sulfate dihydrate into material blocks of 20-50 mm; loading the material blocks into the autoclave through a metal frame or a small car, wherein the autoclave has two types of vertical autoclave and horizontal autoclave; introducing steam condensate or hot flue gas into the autoclave, and heating the material blocks to 50-70° C.; discharging the steam condensate or the hot flue gas, and closing the autoclave; introducing saturated steam into the autoclave, rising a temperature to 120-160° C., and maintaining a pressure; after 5-8 hours of autoclaving, dehydration and crystal transformation, the calcium sulfate dihydrate transforming into the α-calcium sulfate hemihydrate; moving the α-calcium sulfate hemihydrate out of the autoclave and drying, or drying the α-calcium sulfate hemihydrate in the autoclave.
The production cycle with the vertical autoclave is about 16-18 hours, while the production cycle with the horizontal autoclave is about 30-40 hours. The autoclave method is the conventional process and is relatively easy, but has the long production cycle and the relatively high production cost. Moreover, because the material blocks are not uniformly heated, dehydration, crystal transformation and drying of the material blocks are not uniform, causing the large fluctuation of the product quality and the relatively low product strength, wherein the strength is generally about 20-30 MPa.
(2) Hydrothermal Method
Grinding the natural calcium sulfate dihydrate into fine powders, adding the powders into an aqueous solution containing the crystal modifier, and forming a suspension liquid having a solid content not more than 30%; adding the suspension liquid into the vertical autoclave, constantly agitating the suspension liquid, and meanwhile heating the suspension liquid; evaporating water in the suspension liquid, forming steam, generating a pressure, and reaching the autoclaved condition; under the autoclaved condition with the temperature inside the autoclave reaches 120-160° C., reacting for 5-8 hours, and finishing dehydration and crystal transformation; then exhausting air, depressurizing, centrifugally dehydrating, washing, drying, grinding, and finally obtaining the product; crushing and grinding the product, then mixing with water, and forming slurry; adding an additive agent, which is able to facilitate the crystal transformation and also called crystal modifier, into the slurry, then loading the slurry into the autoclave with the steam jacket; agitating the slurry, and meanwhile introducing steam into the jacket and heating; the dihydrate gypsum transforming into the α-hemihydrate gypsum which is well crystallized; exhausting the steam, depressurizing, and discharging the slurry; processing the slurry with dehydration, washing, drying and grinding, and obtaining the high-strength α-hemihydrate gypsum powders.
The α-calcium sulfate hemihydrate prepared through the above method has the stable product quality and the high compressive strength which reaches 40-80 MPa. However, the hydrothermal method has the complex process, requires a few of production equipment and large investment, and has the relatively low production capability, the relatively low production efficiency and the high production cost.
(3) Ordinary Pressure Saline Solution Method
Mixing the ground calcium sulfate dihydrate with the saline solution in which the crystal modifier is added, then adding into the reactor, and boiling under the ordinary pressure condition; the calcium sulfate dihydrate transforming into the α-calcium sulfate hemihydrate; then dehydrating, washing, drying, and obtaining the α-calcium sulfate hemihydrate product. The above method does not require the pressure vessel, which reduces the equipment investment. However, the process condition is strict, the reaction time is relatively long, and the product strength is not high. The method is still in the laboratory testing stage and not able to be applied in the industrialized production.
For the above defects, after a long time of research and practice, the present invention is provided by the inventors. An object of the present invention is to provide a new processing method, which utilizes the calcium sulfate dihydrate to prepare the α-calcium sulfate hemihydrate. The used calcium sulfate dihydrate can be natural raw materials and industrial by-products. The industrial by-products can be directly applied. It is unnecessary to press the industrial by-products to form blocks, add water into the industrial by-products to form a suspension liquid or a saline solution, and separate the solid from the liquid after the reaction is finished. Through directly utilizing characteristics of the industrial by-products, such as the fine particles, the fast heat transfer rate, the uniform heating, the fast reaction rate, and the fast drying rate, the dehydration reaction time and the drying time are shortened, and the product quality is obviously increased.